The activity of Protein C, a vitamin K dependent plasma glycoprotein, is the key element of a naturally occurring, tonically active anticoagulation pathway. The importance of this pathway has been well-documented in recent years by studies demonstrating deficiency of Protein C in inherited and acquired disorders that are characterized by venous thrombosis and/or disseminated intravascular coagulation. Protein C circulates in plasma as a distribution of different molecular species, all potentially precursors to the active enzyme. These structural variants are derived from a single Protein C locus based on genomic maps. Neither the steps involved in processing to the different forms, nor the effect of these changes on function are known. The aims of this proposal include: (1) establishing the nature of the structural differences and their impact on function; (2) determining the normal range of structural and functional variability; (3) determining whether structural or functional differences in Protein C can explain any of the highly variable susceptibility to thrombosis in heterozygous deficiency; and (4) correlating abnormal patterns of variability in acquired disorders to the risk for thrombosis and/or disseminated intravascular coagulation. These goals will be accomplished by (1) analyzing structural variation in plasma samples with specific and sensitive monoclonal antibodies, and by (2) measuring the functional activity of separated molecular forms and of Protein C isolated from individual plasmas in defined systems of activation, expression of activity, and inhibition.